faithful

adj.

c.1300, "full of faith," also "firm in allegiance," from faith + -ful. Meaning "true to the facts" is from 1520s. The noun sense of "true believers" is from 1550s. Related: Faithfully; faithfulness. Old Faithful geyser named 1870 by explorer Gen. H.D. Washburn, Surveyor-General of the Montana Territory, in reference to the regularity of its outbursts.